How to manual disconnect from BLE UART server

Thanks for your response Gordon.

Code changed to:

var gatt;
NRF.connect("30:ae:a4:5d:b3:f2 public").then(function(g) {
  gatt = g;
  return gatt.getPrimaryService("6e400001-b5a3-f393-e0a9-e50e24dcca9e");
}).then(function(service) {
  return service.getCharacteristic("6e400003-b5a3-f393-e0a9-e50e24dcca9e");
}).then(function(characteristic) {
  characteristic.on('characteristicvaluechanged', function(event) {
    
    buf =E.toString(event.target.value.buffer);
    console.log("RX: "+buf);
    if(buf.includes("Quit")){
      gatt.disconnect();
    }
    
  });
  return characteristic.startNotifications();
}).then(function() {
  console.log("Done!");
});

The response after uploading the code varies with changing the code, here 3 examples:

 ____                 _
|  __|___ ___ ___ _ _|_|___ ___
|  __|_ -| . |  _| | | |   | . |
|____|___|  _|_| |___|_|_|_|___|
         |_| espruino.com
 2v06 (c) 2019 G.Williams
>
Done!
RX: D07-06-2020 12.23
RX: SAAPL,311.01,+11.5%
RX: SASML,299.45,-4.5%
RX: Hottentottententente
RX: Quit
RX: D07-06-2020 12.23
RX: SAAPL,311.01,+11.5%
RX: SASML,299.45,-4.5%
RX: Hottentottententente
RX: Quit
Uncaught Error: BLE task 3 is already in progress
 at line 1 col 107
...s("Quit")){gatt.disconnect();}
                              ^
in function called from system
RX: D07-06-2020 12.23
RX: SAAPL,311.01,+11.5%
> 

 ____                 _
|  __|___ ___ ___ _ _|_|___ ___
|  __|_ -| . |  _| | | |   | . |
|____|___|  _|_| |___|_|_|_|___|
         |_| espruino.com
 2v06 (c) 2019 G.Williams
>
Done!
RX: D07-06-2020 12.23
RX: SAAPL,311.01,+11.5%
RX: SASML,299.45,-4.5%
RX: Hottentottententente
RX: Quit
RX: D07-06-2020 12.23
RX: SAAPL,311.01,+11.5%
RX: SASML,299.45,-4.5%
RX: Hottentottententente
RX: Quit
Uncaught Error: Not connected
 at line 1 col 107
...s("Quit")){gatt.disconnect();}
                              ^
in function called from system
>

 ____                 _
|  __|___ ___ ___ _ _|_|___ ___
|  __|_ -| . |  _| | | |   | . |
|____|___|  _|_| |___|_|_|_|___|
         |_| espruino.com
 2v06 (c) 2019 G.Williams
>
Uncaught Error: Unhandled promise rejection: Disconnected
>

Not sure what is happening in the background.